An interest in environmental issues has been steadily growing in recent years, and demand has accordingly been created for weight reduction based on stress increase and wall thickness reduction in a variety of machined parts, primarily automotive parts. Further, since processing in various deformation modes, such as press working and stretch flanging, is performed, steel sheets are required to have high ductility and hole-expandability in addition to strength. Furthermore, from the standpoint of extending service life and eliminating post-coating, high-strength rust-resistant steel sheets are needed.
Patent Literature 1 (PTL 1) discloses a hot-dip coated steel sheet with high strength and hole-expandability. However, since the strength is ensured by using a bainitic structure, which is a hard phase, the hole-expandability is high, but ductility is low.
Patent Literature 2 (PTL 2) suggests a ferritic single-phase steel sheet as a material with increased ductility and hole-expandability. However, since Mo is contained, the cost is significantly increased.
Patent Literatures 3 and 4 (PTL 3 and 4) suggest a multiphase steel sheet, in which the difference in hardness between ferrite and martensite is reduced, as a material with increased ductility and hole-expandability. However, since the martensite hardness is decreased, where the ferrite volume ratio is increased, the strength is reduced. Further, the hole-expandability is not necessarily high.
Patent Literature 5 (PTL 5) suggests a steel sheet hot-dip coated with a Zn—Al—Mg-based system and excelling in hole-expandability and corrosion resistance. However, it was found that high hole-expandability is not always easy to obtain even by following the method disclosed in this literature.